chore: add Biome and document design decisions from grill session

- Add @biomejs/biome 2.5.3 with 2-space indent, double quotes, trailing commas
- Add lint and format scripts to root package.json
- Update CONTEXT.md: resolve Structured Output shape, add Document AST, Block, Inline Node terms
- Add ADR 0002 (remark as internal Markdown parser)
- Add ADR 0003 (core defines own AST types)
- Add spec: core-parser (Parser implementation and Document AST)
- Add spec: cli-view (kitchen view <file> command)

biome.json

@@ -0,0 +1,38 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.5.3/schema.json",
+  "vcs": {
+    "enabled": true,
+    "clientKind": "git",
+    "useIgnoreFile": true
+  },
+  "files": {
+    "ignoreUnknown": false
+  },
+  "formatter": {
+    "enabled": true,
+    "indentStyle": "space",
+    "indentWidth": 2,
+    "lineWidth": 100
+  },
+  "linter": {
+    "enabled": true,
+    "rules": {
+      "preset": "recommended"
+    }
+  },
+  "javascript": {
+    "formatter": {
+      "quoteStyle": "double",
+      "trailingCommas": "all",
+      "semicolons": "always"
+    }
+  },
+  "assist": {
+    "enabled": true,
+    "actions": {
+      "source": {
+        "organizeImports": "on"
+      }
+    }
+  }
+}